It's a 500mm lens. That means that on a cropped sensor, it's equivalent to 750mm and to get sharp results you'll need a shutter speed of at least 1/750s. If you look at all his shots, this is the reason he pushes high ISO.

read the entire thread. The fact is, that 500mm works out to 750mm with the crop factor, so you need minimum 750th of a second shutter speeds to stop camera shake. This lens doesn't have IS/VR etc. Because of that, you have to make a choice shoot lower Iso and get camera shake, or shoot higher and get noisey but shake free shots.